With the final stage fast approaching in the long and grueling Progressive Auto X-Prize competition, it’s now up to just one team to hold things together long enough to claim $5 million in cash, organizers have announced.

The X-Prize, designed to prove it possible to produce an attractive and affordable mainstream car capable of getting 100 miles per gallon may be demonstrating exactly the opposite. One after another, promising designs and technologies simply haven’t been able to live up to the challenges set by organizers who were hoping to show that lean and green automobiles are ready for prime time.

Following the past week’s shake-out, with only a few hurdles left before the awards are formally announced, in Washington, D.C., in September, two vehicles survive in the tough mainstream class. Both are fielded by the Edison2 team, which is led by European car dealer Oliver Kuttner, and brings together an assortment of motorsports and defense specialists.

But there’s still a bitter battle underway between more than a dozen teams hoping to claim a pair of $2.5 million prizes for alternative, high-mileage vehicles.

Modeled after the Orteig Prize, which helped motivate Charles Lindbergh’s legendary trans-Atlantic flight – and the Ansari X-Prize which spurred the first private manned spaceflight, in 2004 – the Progressive Auto X-Prize was designed to encourage the development of clean, high-efficiency vehicles.

Alternatives!

But the goal, stresses Eric Cahill, an energy researcher and now the senior director of the Auto X-Prize, has been to do more than just reward a brilliant piece of technology that could never make it into production. Each of the 115 teams that originally signed up for the competition has not only had to meet strict technical mandates, but also prove that it could mass produce its entry at an affordable price — and demonstrate why the offering would actually generate a market.

Ironically, when the project was first announced, in April 2008, “We were getting criticized for making it seem too easy,” says Cahill. “Many thought that getting the equivalent of 100 miles per gallon wouldn’t be a problem.”